The present invention relates generally to an apparatus and method for tuning oscillator and filter alignment. More specifically, the invention relates to an apparatus and method tracking the high frequency filters and local oscillator, used for example in TV tuners, and similar systems working over a wide frequency range such as terrestrial and cable TV broadcasting (40 to 860 MHz), satellite TV first IF (950 to 2400 MHz), shortwave receivers (2 to 30 MHz), FM broadcast receivers (88 to 108 MHz, etc.
Typical TV tuner architectures usually consist of a variable gain pre-amplifier stage with a single tuned resonant circuit at the input. The variable pre-amplifier stage is typically a dual-gate MOSFET. The output of the variable gain pre-amplifier stage is connected to a mixer stage via a tuned bandpass filter, which may comprise of two coupled parallel LC circuits. The mixer is also driven by a signal derived from a local oscillator with a frequency that is determined by a tuneable resonant circuit. The tuning of the filters and the local oscillator is usually achieved by variable capacitance diodes such that the same control voltage is applied to all of the variable capacitance diodes generated by a phase-locked-loop arrangement. The role of the filters is to reduce the level of unwanted signals to avoid overloading of the receiver and in particular any signal falling within the image frequency response created by the mixing of the input signals to an intermediate frequency. The rejection of the image frequency response is of considerable importance when the intermediate frequency is low and the image frequency falls with the band to be covered by the system. In order to cover the entire frequency range, for example to 860 MHz in terrestrial and cable TV broadcasting, the range is typically divided by two or three bands, with each band covering one or more octaves. Each band then usually requires an input filter, gain controlled amplifier, bandpass filter, mixer and local oscillator, where the mixers, local oscillators and phase-locked-loop circuit are usually combined in one or more integrated circuits.
Another means of covering the wide frequency range consists in mixing the incoming signals to create an image frequency outside the band to be received. It is then possible to reject the image frequency response with no-tuneable lowpass or highpass filters. However, this method does not protect the receiver from strong unwanted signals within the band to be received. A tuneable filter at the input is often required to this end. With these typical configurations, tracking over the desired band is achieved by the use of matched variable capacitance diodes and by factory adjustment of the coils on a few carefully chosen channels, for example low end, high end, and centre of band.
However, with these approaches errors can occur on channels between the adjustment points, often reaching a maximum midway between them. One method to compensate for this and to reduce any degradation of performance due to mis-tracking, is to design the filters with a passband wider than required, so that any tuning errors will cause a negligible degradation of the received signal. Associated errors in tracking caused by drift with temperature have been minimised with careful design by using matched variable capacitance diodes. In order to reduce the number of external components, there is a need to integrate all the frequency determining components of the local oscillator. However, it is difficult to achieve this by using the same tuning voltage for the filters and the local oscillator fully integrated within an integrated circuit, since the characteristic of the local oscillator tuning voltage will then be different to that of the filters. If the tuning characteristics are known and stable, it is possible to generate the tuning voltage by any known analogue or digital means. However, it is difficult to compensate for different temperatures with mechanical and electrical design when the local oscillator is completely integrated. For example, in U.S. Pat. No. 4,736,456 a predetermined voltage is added to the local oscillator tuning voltage to obtain a filter tuning voltage, however, as the frequency determining elements of the local oscillator are within an integrated circuit, variation of frequency with temperature of the local oscillator will differ from that of the filters. Furthermore, the frequency determining elements of the filters will undergo mechanical and electrical stresses which are largely different from those within the integrated circuit, causing ageing.
Thus, there is a need for an apparatus and method for tuning the frequency of the local oscillator and of the filters for applications where filter tuning signal is generated independently of the local oscillator tuning signal in a wide tuning range, particularly when some of elements are completely integrated within an integrated circuit.
An apparatus and method are provided for tuning a filter for applications where the filter tuning signal is generated independently of the local oscillator tuning signal and the tuning range is large. The filter receives an input signal and provides an output signal. The filter is adapted to receive a tuning control signal to adjust the filter to provide the output signal to a desired signal. The tuning control signal results from a modulation signal, a correction signal and an approximate filter tuning signal generated by a filter tuning signal generator. The modulation signal has a frequency that varies between a first frequency and a second frequency, a first value of the output signal measured at the first frequency, and a second value of the output signal measured at the second frequency. A comparator compares the first value and the second value and provides the correction signal to adjust the filter with the tuning control signal with respect frequency and the second frequency of the modulation signal, the correction signal and the approximate filter tuning signal.